If I understand correctly, 便利さ and 利便性 have the same meaning.
Is there any difference in nuance or usage?
Maybe 利便性 is more formal?
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便利 is a quality something provide, 利便 is a quality people perceive.
It's true that 利便性 sounds more formal than 便利さ, but it mainly comes from the difference between -性 and -さ. Theoretically, 便利 also has a form of 便利性 as well as 利便 has 利便さ, but apparently disfavored than their counterparts according to Google search.
便利(な) can modify almost anything; 調理器具, 文房具, ソフトウェア, (何でも売っている)店, (何でも答えてくれる)ウェブサイト, (色々な状況で使える)ことわざ, (頼みを断らない)友人, etc. (Don't use the last example unless you want to lose your friends.) Commodities such as 鉛筆 or 包丁 are almost exclusively modified by 便利.
利便性の高い tends to modify larger and/or public things, mainly focusing on their accessibility rather than functionality. For example, (駅から徒歩で1分の)家, (24時間営業の)店, (大都市を短時間で結ぶ)路線, (簡単な手続きで使える)公衆無線LANサービス, etc. (But you can use 便利 for these, too)
And 利便性 does sound more formal; it's rarely used in casual conversations.
After the discussion below and Google search, I think 利便性の高い家 and 便利な家 well illustrated the difference of the two words.
Quite contrary to Kentaro Tomono's statement, apparently 利便性の高いマンション almost always refers to its location -- good access to the nearby facilities such as a station, supermarket, school and library.
It was not until I checked the 57th Google result that I could find this term used for a different aspect of residency.
便利なマンション also refers to the same thing in most cases (e.g. 通勤に便利な家). But it was far easier for me to find different examples, such as 家具付きのマンション, エアコン完備のマンション, あると絶対便利なマンションの施設.